As a method of forming an oxide film, such as a tunnel barrier layer of a TMR multilayered film, required to have a very small uniform film thickness of about 0.5 to 2 nm, a method of forming a thin metal film having a predetermined film thickness and forming a barrier layer having a desired thickness by oxidizing the metal film is adopted. Since the magnetoresistance change rate and junction resistance of the TMR multilayered film fluctuate in accordance with, for example, the barrier layer formation method and film quality, various oxidizing methods capable of increasing the magnetoresistance change rate and decreasing the junction resistance have been studied.
In Japanese Patent Laid-Open No. 2001-57450, for example, a nonmagnetic metal oxide layer in a TMR multilayered film including a first ferromagnetic layer/nonmagnetic metal oxide layer (barrier layer)/second ferromagnetic layer is formed by forming a thin metal film and performing plasma oxidation, natural oxidation, or radical oxidation, and the magnetoresistance change rates, the junction resistances, and the like of the obtained TMR multilayered films are compared and examined.
Consequently, in plasma oxidation in which the thin metal film is exposed to a plasma, the oxidation rate is very high, and even the ferromagnetic layer positioned below the barrier layer suffers oxidation damage. This reveals the problem that it is not easy to stably form a multilayered film having a low resistance value. To avoid this problem, Japanese Patent Laid-Open No. 2000-36628 has disclosed a barrier layer formation method that repeats the formation of a thin metal film and plasma oxidation twice or more. Unfortunately, this method has the problem that the number of manufacturing steps increases and the productivity decreases. Also, the natural oxidation method makes it possible to obtain a resistance two orders of magnitude or more lower than that obtained by the plasma oxidation method, but has the problems that the productivity decreases because an oxidation time of 60 min or more is necessary, and an unoxidized portion that significantly deteriorates the TMR characteristics readily forms.
On the other hand, compared to the natural oxidation method, radical oxidation has the advantage that although the junction resistance increases, the productivity increases and a high resistance change rate can be obtained. Also, compared to the plasma oxidation method, radical oxidation has the advantage that a high-performance TMR multilayered film can be stably produced.
Unfortunately, the radical oxidation method is still unsatisfactory and has its limit in order to further decrease the junction resistance and further increase the resistance change rate. That is, the radical oxidation method has an oxidizing power higher than that of the natural oxidation method, but this oxidizing power is still insufficient. As a consequence, an unoxidized portion sometimes forms, and this makes it difficult to improve the characteristics of the TMR multilayered film.
Instead of the oxidation methods described above, it is also possible to perform oxidation by using an oxygen ion beam by applying an apparatus disclosed in, for example, Japanese Patent No. 3159097. In this method, a plasma chamber for generating a plasma and a processing chamber containing a substrate are coupled via a mesh-like extracting electrode, an ion beam is extracted from the plasma by applying a voltage to the extracting electrode, and the substrate surface is oxidized by this ion beam. In a processing apparatus like this, however, the ion beam oxidizing apparatus using the mesh-like partition cannot sufficiently isolate the plasma, and inflicts damage to the oxide film as in plasma oxidation. This makes a TMR multilayered film having stable characteristics impossible to obtain.